The present embodiments relate generally to methods and compositions for cementing. In certain embodiments, methods and compositions for cementing in a subterranean zone penetrated by a wellbore are described.
Conventionally, a wellbore is drilled into a subterranean zone using a drilling fluid that is continuously re-circulated downwardly through the interior of a drill pipe and upwardly through the annulus between the exterior of the drill pipe and the walls of the wellbore. After a wellbore has been drilled to total depth, the circulation of the drilling fluid is stopped (called a “shut-down period”), the well is logged and casing is run in the wellbore.
After casing is placed in the wellbore, drilling fluid is again continuously re-circulated downwardly through the interior of the drill pipe and upwardly through the annulus between the exterior of the pipe and the walls of the wellbore, to remove drilling solids, filter cake, dehydrated drilling fluid, gelled drilling fluid, cuttings, and other debris.
However, the polymeric viscosifiers and additives typically used in drilling fluids create a filter cake that is generally very stable and difficult to remove. In addition, additives typically used in conventional drilling fluids, primarily bentonite, cause the drilling fluid to be incompatible with cementing compositions, such that if a cementing composition comes into contact with an appreciable amount of a conventional drilling fluid, the cementing composition will gel and become unpumpable.
Thus, removal of filter cake from the walls of the wellbore, and displacement of the drilling fluid from the wellbore, must take place prior to cementing the casing in the wellbore. If an appreciable amount of drilling fluid and/or filter cake remain in the annulus or on the walls of the wellbore, a cementing composition pumped into the wellbore can gel and become unpumpable, and/or the cementing composition will not properly bond to the walls of the wellbore and the casing. To remove drilling fluid and filter cake from the wellbore, it is known to run flushes, spacer fluids, and fluids at high turbulence, through the annulus between the casing and the walls of the wellbore prior to cementing.
With the wellbore cleared of drilling fluid and/or filter cake and/or other debris, the casing is cemented in the wellbore by placing a cementing composition in the annulus between the casing and the walls of the wellbore. The cementing composition is prepared by mixing dry cementitious material with a mixing fluid, and pumping the cementing composition down the interior of the casing and upwardly through the annulus between the exterior of the casing and the walls of the wellbore. The cementing composition sets into a hard impermeable mass, and is intended to bond the casing to the walls of the wellbore whereby the annulus is sealed and fluid communication between subterranean zones or to the surface by way of the annulus is prevented.